High frequency magnetron



July 19, 1955 E. D. MCARTHUR 1 ,6

HIGH FREQUENCY MAGNETRON Filed Jan. 18, 1951 Inventor:

Elmer- D. McAT-thur,

b3 2?! 4. fillwb His Attorney.

United States Patent 715,653 incn FREQUENCY MAGNETRON Elmer D. McArthur,Scotia, N. Y., assignor, by mesne assignments, to the United States ofAmerica as represented by the Secretary of the Navy Application January18, 1951, Serial No. 206,611 Claims. (Cl. 315-39-53) My inventionrelates to and, more particularly, known as magnetrons.

In the past few years, applications in the electronics art have mademore and more use of ultra-high and super-high frequency oscillations,and the magnetron has been developed and used as one of the mostimportant means for generating such high frequency oscillations fromdirect current electrical energy. Essentially, all magnetrons in use atthe present time employ a cylindrical cathode within a concentric anode,a radial electric field set up between these electrodes by a differenceof potential applied to them, and an axial magnetic field established inthe space between the electrodes by a permanent or electromagnet. Wellknown classifications of magnetrons, according to principle ofoperation,

electron discharge devices to such vacuum tube devices are the negativeresistance, electron transit-time, and car;

traveling wave types, which depend mainly upon differences in anodeconstruction and resonating means for their diiferences in principle ofopeartion. It has become well known that the traveling wave typemagnetron, commonly known also as the multicavity magnetron, employingan anode block having several resonant cavitiesand an output couplingloop or other coupling means is the best suited to generate super-highfrequencies with a reasonable degree of efliciency. Thus, themulticavity magnetron its theory of operation is well known to the art.

However, at super-high frequencies the amount of power and length oftube life that may be obtained from a given multicavity magnetron arestill limited, especially at the upper super-high frequencies, bypossible destruction of the cathode, since the emitting surface isrelatively small, resulting in high current density. Further, aconsiderable cathode back heating effect is encountered, due toelectrons returning to and striking the cathode. been increased in orderto enlarge the cathode emitting surface, but, for the upper super-highfrequencies where the size of resonant cavities is necessarily small dueto the short wave lengths, the number of resonant cavities has come intowide usageand space near the free ends of these pole pieces by flaringThe diameter of the cathode and anode has L required around the innersurface of the anode became so great that transient and spurious modesof oscillation, that is, instability of frequency, resulted. Like wise,the axial length of the electrodes has been increased in order toenlarge the cathode emitting surface, but this also resulted ininstability of frequency, due to transient and spurious modes ofoscillation of space charge along the axial dimension. Thus, at thepresent time, multicavity magnetrons having a reasonable life arelimited in power output at the upper super-high frequencies.

It is, therefore, an object of my invention to provide a magnetron foroperation at higher frequencies.

It is a further object of my invention to provide a magnetron havinglonger life and higher power output.

In carrying out my invention, I provide a vacuum tube electron dischargedevice having opposed parallel pling loop, are positioned 2,713,653Patented July 19, 1955 2 disk electrodes, the anode having in onesurface radial slots of proper depth and shape to form resonant cavitiesat a desired frequency, and a magnet to establish a radial magneticfield in the space between the electrodes. Suitable output means, suchas an output couproperly within one of the resonant cavities in a wellknown manner. When a difference of potential is applied to theelectrodes, an axial electric field is set up in the space between theelectrodes, and the device generates high frequency oscillations. Sincethe cathode emitting area is essentially as large as the anode area, asopposed to a smaller emitting area in conventional magnetrons, morecurrent may be drawn from the cathode at the same current density andany back heating is distributed over a ice larger area.

For a better understanding of my invention, together with furtherobjects and advantages thereof, reference should now be had to thefollowing description referring to the accompanying drawing, in which:

Fig. 1 illustrates, by a partial sectional view, an electron dischargedevice embodying my invention; Fig. 2 is a side elevation of thepermanent magnet shown by sectional view in Fig. 1; Fig. 3 is aperspective view of the anode shown in Fig. 1; and Fig. 4 is aperspective view of another embodiment of the anode which may be used inmy invention.

Referring to Fig. 1 of the drawing, an electron discharge device of themagnetron type is shown comprising an outer envelope 1, made of metalfor the preferred embodiment illustrated, defining an evacuated chamberin which are mounted a permanent magnet 2, a metallic annular diskforming a cathode 3, and a metallic block forming an anode 4.

The permanent magnet 2 is further illustrated by an end view in Fig. 2and comprises a shaft-like center pole piece 2a made integral with orsecured to a cylindrical outer pole piece 2b through a disk portion 20.The radial magnetic field between inner pole piece 2a and the outer polepiece 2b may be concentrated in the the free end of pole piece 2a, asshown in Fig. 1.

The cathode 3 is supported from an annular disk 5, made of suitableinsulating material, which is secured, as shown, to the pole pieces 20:and 2b. The cathode 3, as shown in a preferred form, is made of asuitable metal, such as nickel, to form a hollow annular disk containinga heater element 6 terminated by conductors 7 external tothe envelope 1.A conductor 8 is connected to the cathode 3 and also terminatedexternalto the envelope 1. The surface 3a is made of, or coated with, a suitablematerial, such as an oxide impregnated nickel matrix, to emit electronswhen thermally excited by heater element 6. Concentric cathode shields3b are metallic extensions of cathode 3 provided to prevent electronsemitted by the cathode surface 3a from reaching the permanent magnet 2or the envelope 1, thereby limiting the destination of emitted electronseither to the anode 4 or the cathode 3. Shields 3b correspond to thefamiliar end shields commonly employedin conventional magnetrons.

The anode 4 is secured to one end of envelope 1 by suitable connectingmeans, as for instance by welding, so that the two are always at thesame electric potential. Radial resonant cavities 4a are provided in theface of the anode 4 which faces the cathode 3 and a circular depression4b permits the pole piece 2a and one shield 3b to extend thereinto asshown in Fig. l. A coupling loop 9, is disposed in one of the cavities4b and connected by a conductor 10 external to envelope 1 to provideoutput means for the device. Other well known output means, such as anoutput wave guide coupled to one of the cavities 4!), may be employed.Suitable insulating seals,

such as glass beads 11, are provided to permit the conductors 7, 8 andto extend from within envelope 1 while a positive vacuum seal ismaintained.

Fig. 3, by a perspective view, shows more clearly the construction ofanode 4 and the radial resonant cavities 4a formed in one face thereof.The width and depth, as well as the number, of resonant cavities 4a isdetermined in the design of the anode by the desired frequency ofoscillation to be obtained from the magnetron. The cavities 4a, however,are not limited to the slot type shown by Fig. 3 but may be of the slotand hole type shown in Fig. 4 or may be of the familiar rising sunconfiguration. Strapping means, such as straps 12 in Fig. 4, may beprovided around the curved surface of the anode 4 to connect alternatesegments and, thus, to stabilize the mode of oscillation.

In operation, which may be pulsed operation, the cathode 3 is maintainedat a direct current potential negative with respect to the groundedenvelope 1 and anode 4. This establishes an axial electric field betweenanode 4 and cathode 3. A current is supplied to heater element 6 throughconductors 7 to heat surface 3a and, thus, to cause it to emitelectrons. Electrons emitted from surface 3a move under the combinedinfluence of the axial direct current electric field and the radialmagnetic field to produce electromagnetic fields varying at radiofrequency within the resonant cavities 4a. While the paths of electrontravel are not the same as those paths in a conventional magnetron, thebasic principle of operation, which causes a net transfer of energy fromthe direct current field to the radio frequency fields is similar innature to that of the conventional magnetron.

It will be readily apparent that the emitting surface 3a is as large orlarger in area than the anode receiving surface. Therefore, the emissioncurrent densities for a given power output need not be as high, norcathode overheating due to electron bombardment as likely, as in theconventional magnetron. Thus, the magnetron of my invention may beconstructed to have a reasonable power output and tube life at the uppersuper-high frequencies, and greater power output than presentlyavailable at lower frequencies.

The magnetron of my invention may be provided with water coolingfeatures, as are well known, to further increase its power outputcapacity and life. It is conceivable that the envelope 1 of my inventionmay, in some applications, preferably be made of glass. This may be doneby providing a separate lead from the anode 4 through the envelope 1 tothe exterior thereof. Also by shaping the envelope 1 properly with adeep depression in one end, the permanent magnet 2 may be removablypositioned external to the envelope 1 so that it can be interchangedwith other magnets providing different magnetic field strengths.

While the present invention has been described by reference to aparticular embodiment thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the invention. I, therefore, aim in the appended claimsto cover all such equivalent variations.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electron discharge device of the magnetron type comprising anevacuated envelope, an annular disk cathode therein, means to heat saidcathode, a circular anode in spaced relation with said cathode providedwith resonating means therein, output coupling means in conjunction withsaid anode, and means disposed within the evacuated envelope forestablishing a radial magnetic field in the space between said cathodeand said anode.

2. An electron discharge device of the magnetron type comprising anevacuated metallic envelope, an annular disk cathode therein, means toheat said cathode, a circular block anode connected to said envelope inspaced relation with said cathode and having one face thereof defining aplurality of radially oriented resonant cavities, an output couplingloop positioned in one of said cavities, and a permanent magnet disposedwithin the evacuated envelope for providing a radial magnetic field inthe space between said cathode and said anode.

3. An electron discharge device of the magnetron type comprising anevacuated metallic envelope, an annular disk cathode therein, twoconcentric and cylindrical shields on said cathode, a heater elementpositioned within said cathode, a cylindrical anode connected to saidenvelope in spaced relation with said cathode and having one facethereof containing a plurality of radially oriented resonant slots, anoutput coupling loop positioned in one of said slots, 21 permanentmagnet disposed within the evacuated envelope for providing a radialmagnetic field in the space between said cathode and said anode, vacuumseals in said envelope, and conductors extending through said seals fromsaid cathode, said heater element, and said coupling loop.

4. An electronic discharge device of the magnetron type comprising anevacuated envelope, a permanent magnet within said envelope andcomprising a disc portion, a central pole piece portion and an outercylindrical pole piece, said central pole piece and said cylindricalpole piece being substantially coextensive, an annular cathode aboutsaid central pole piece and within said outer cylindrical pole piece, ananode axially spaced from said cathode and central pole piece, aninsulating disc between said disc portion of said magnet and saidcathode, cathode shields extending from said cathode in proximity tosaid central pole piece, and end shields inside said cylindrical polepiece in proximity thereto.

5. An electron discharge device as defined in claim 4 but furthercharacterized by said insulating disc and said end shields being incontact.

References Cited in the file of this patent UNITED STATES PATENTS2,270,777 Von Baeyer Jan. 20, 1942 2,411,953 Brown Dec. 3, 19462,412,824 McArthur Dec. 17, 1946 2,437,279 Spencer Mar. 9,1948 2,443,179Beniotf June 15, 1948 2,485,401 McArthur Oct. 18, 1949

